1. Field of the Invention
The present invention relates to an electrophotographic image forming process using an electrophotographic photoconductor, and more particularly to an electrophotographic image forming process comprising the steps of charging, light exposure, reversal development, and image transfer, using an electrophotographic photoconductor comprising an electroconductive support and a photoconductive layer formed thereon comprising a residual solvent.
2. Discussion of Background
The Carlson process and other processes obtained by modifying the Carlson process are conventionally known as the electrophotographic methods, and widely utilized in the copying machine and printer. In a photoconductor for use with the electrophotographic method, an organic photoconductive material is now widely used because such an organic photoconductor can be manufactured at low cost by mass production, and causes no environmental pollution.
Many kinds of organic photoconductors are conventionally proposed, for example, a photoconductor employing a photoconductive resin such as polyvinylcarbazole (PVK); a photoconductor comprising a charge transport complex of polyvinylcarbazole (PVK) and 2,4,7-trinitrofluorenone (TNF); a photoconductor of a pigment dispersed type in which a phthalocyanine pigment is dispersed in a binder resin; and a function-separating photoconductor comprising a charge generation material and a charge transport material. In particular, the function-separating photoconductor has now attracted considerable attention.
The mechanism of the formation of latent electrostatic images on the function-separating photoconductor is as follows:
When the photoconductor is charged to a predetermined polarity and exposed to light, the light passes through a transparent charge transport layer, and is absorbed by a charge generation material in a charge generation layer. The charge generation material generates charge carriers by the absorption of light. The charge carriers generated in the charge generation layer are injected into the charge transport layer, and move in the charge transport layer depending on the electric field applied to the photoconductor at the charging step. Thus, latent electrostatic images are formed on the surface of the photoconductor by neutralizing the charge thereon. As is known, it is effective that the function-separating electrophotographic photoconductor employ in combination a charge transport material having an absorption intensity mainly in the ultraviolet region, and a charge generation material having an absorption intensity mainly in the visible region.
As the charge transport materials, many low-molecular weight compounds have been developed. However, the film-forming properties of such a low-molecular weight compound are very poor, so that the low-molecular weight charge transport material is dispersed and mixed with an inert polymer to prepare a charge transport layer. The charge transport layer thus prepared using the low-molecular weight charge transport material and the inert polymer is generally so soft that the charge transport layer will easily peel off during the repeated electrophotographic operations by the Carlson process.
The electrophotographic process using the above-mentioned electrophotographic photoconductors has made remarkable progress in recent years. Namely, the conventional Carlson process has been improved from various aspects. For instance, improvement in the cleaning step to completely cancel the previous record of image forming operation is described in Japanese Laid-Open Patent Application 58-102273; improvement in the charging step to reduce the amount of generated ozone is described in Japanese Laid-Open Patent Applications 56-104351, 57-178267, 58-40566, 58-139156, 58-150975, and 63-149669; and improvement in the image transfer step to upgrade the image quality of the obtained image is described in Japanese Laid-Open Patent Applications 5-45916 and 7-152217.
However, in the electrophotographic process including the reversal development, the charge with a polarity opposite to the polarity obtained by the charging step is necessarily applied to the photoconductor in the image transfer step. As a result, charge carriers are easily trapped at the interface between the undercoat layer and the charge generation layer, between the charge generation layer and the charge transfer layer, or between the photoconductive layer and the protective layer, in the electrophotographic photoconductor. In particular, it is considerably difficult to remove the charge carrier with the opposite polarity once accumulated at the above-mentioned portions. In the electrophotographic process including the reversal development, the charge with the opposite polarity is applied to the photoconductor at the image transfer step, and the charge carriers are readily trapped in the portion of the photoconductor which is subjected to image transfer charging.
As a result, there are caused various problems. One of the problems is that the charging potential and the potential after light exposure are unfavorably decreased in the repeated electrophotographic operations, with the result that the obtained character images become thicker.
Further, the above-discussed trapped charge carriers tend to easily gather at the edge portion of an image transfer sheet. Therefore, there is a risk of a stripe image abnormally appearing on the image transfer sheet at the edge portion thereof, perpendicular to the transporting direction of the transfer sheet. To solve the above-mentioned problem, it is proposed that the photoconductor be charged to the opposite polarity with respect to the polarity of the image transfer charging after the completion of image transfer step in order to compulsorily cancel the charge carriers by means of the applied electric field.
In addition, there is proposed an electrophotographic image forming apparatus comprising a means for removing the trapped charge carriers by simultaneously carrying out the light exposure step and the charging step, which means is provided between the image transfer means and the charging means (Japanese Laid-Open Patent Application 8-262941). However, in the case where the energy level of the trapped carrier is deep, the above-mentioned proposal is not effective. Further, since the light exposure and the charging are simultaneously carried out to cancel the trapped carrier, the photoconductor suffers electrostatic fatigue, with the result that the durability of the photoconductor is impaired.
Furthermore, because of the above-discussed trapped charge carriers, there occurs a troublesome problem. The problem is that the charging potential obtained in the first rotation of the photoconductor is different from the charging potential obtained after the photoconductor is rotated several times. In other words, the charging potential is not stabilized until the rotation of the photoconductor is repeated a plurality of times. In particular, a slight change in charging potential seriously affects the image quality in the electrophotographic color image forming process. To be more specific, formation of a color image is completed through a plurality of rotations of the photoconductor. If there is a change in charging potential, color reproduction cannot be exactly performed. In addition, when a plurality of copies is made, the image density becomes uneven.
This phenomenon is considered to depend upon the number of sheets which has been subjected to copying operation, and the time when the photoconductor is allowed to stand before the image formation is started again. The shorter the intermission, the more serious the phenomenon. The time required to stabilize the charging potential varies depending upon the amount of trapped carries and the time spent to release the trapped carriers.
As countermeasures against the above-mentioned phenomenon, it is proposed that the image forming process by the first rotation of the photoconductor be ignored, and the image forming processes by the second rotation and sequential rotations thereto be adopted in practice because the charging potential becomes stable after the second rotation. However, this is an obstacle to high-speed operation of the digital copier. In addition, the above-mentioned proposal cannot perfectly solve the problem in light of the factor of the intermission time before the image formation is started again.
Thus, there is proposed a photoconductor comprising a photoconductive layer which comprises a phthalocyanine compound, and an undercoat layer which is provided under the photoconductive layer and comprises a semiconductor material with a band gap of 2.2 eV or more and a binder resin (Japanese Laid-Open Patent Application 10-186703). The effect of this proposal is still unsatisfactory.